seL4RPC: Fix prototype for user functions that return an array.
seL4RPC: Fix prototype for user functions that return an array.

This was an oversight in a previous glue code refactor. Note that this should
not have caused users any difficulty as array return values are not supported
at the parser level currently.

smaccm/camkes-tool,smaccm/camkes-tool,agacek/camkes-tool,smaccm/camkes-tool,agacek/camkes-tool,smaccm/camkes-tool,agacek/camkes-tool